Add python-debianbts as a dependency in setup/README (Closes: #823118)

Needed to run the test suite:

    ======================================================================
    ERROR: test_importing_rm (test_imports.ImportTestCase)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/srv/daksrc/tests/test_imports.py", line 15, in test_fn
        __import__('dak', fromlist=[cmd])
      File "/srv/daksrc/dak/rm.py", line 54, in <module>
        from daklib.rm import remove
      File "/srv/daksrc/dak/daklib/rm.py", line 52, in <module>
        import debianbts as bts
    ImportError: No module named debianbts
Signed-off-by: NLuke Faraone <lfaraone@debian.org>

setup/README

@@ -6,7 +6,7 @@ The following packages are needed for the database:
 and the following packages for dak itself:
  * python-psycopg2 python-sqlalchemy python-apt gnupg dpkg-dev lintian
    binutils-multiarch python-yaml less python-ldap python-pyrss2gen python-rrdtool
-   symlinks python-debian
+   symlinks python-debian python-debianbts
 
 (the schema assumes at least postgresql 9.1; ftpmaster in Debian currently uses
 the postgresql 9.4 version from Debian 8)